The Sportster .28 I have used a conversion head from Davis Diesel Development . It would start and run fine. As you stated, it must not be over compressed, and plenty of ether and castor oil is needed. The Sportster connecting rod, cylinder/case and piston used a high silicon content aluminum alloy which worked OK for glow use but was not as good as bronze bushings. Without bushings, it did not last nearly as long as a diesel than it would have lasted as a glow before it got real loose. Never broke a rod, just wore out the journals. One way to tell if a diesel is over compressed is to look at the exhaust. It it is black (due to soot) then it is over compressed. Normally, when run at the correct compression setting and with fuel containing an adequate amount of ether, the engine will have a slight miss or skip occasionally and the exhaust residue should look like clear honey.

As for recommending engines designed as diesels rather than conversions, I have had better success with engines specifically designed as diesels. I have two PAW 1cc ball race RC engines that are little gems. Easy to start and will idle down to <2500 rpm, even when mounted inverted. I also have a MP Jet 1cc diesel that absolutely screams. It turns an APC 7x3 at 16,800 and makes over 0.2 BHP. Not bad for a 1cc engine (.061). The nice thing about diesels is they will idle reliably inverted just as well as upright since there is no glow plug to quench if the idle mixture is too rich. With a diesel, if the idle mixture is too rich and fuel starts accumulating, it just boosts the compression hydraulically so that it still fires.

My biggest problem with diesels is getting good fuel locally and keeping it fresh. I keep having to add di-ethyl ether to "freshen" the fuel to where it will start and run without being over compressed. Getting good ether is not as easy as it used to be thanks to all the meth labs.. Now I have to get it from John Deere as cans of starter fluid which also has butane/heptane in it too. Also, the amyl nitrate or isopropyl nitrate typically added as an ignition improver (about 2-3%) is hard to obtain because they are chemically similar to nitro glycerin and BATF takes a dim view of modelers having it. However, Diesel Clean Cetane Booster (available at Walmart) can be substituted at 4% instead.